Until now, to prevent the occurrence of cavitation in a water jet driven personal water craft, the number of revolutions of a water jet pump is controlled. For an example, refer to JP-A-2001-328591, which discloses an invention for avoiding cavitation in a water jet boat without depending upon the experience and intuition of a pilot. According to this invention, a water jet pump is operated based upon the practical target number of revolutions and the actual number of revolutions by calculating the actual number of revolutions of a water jet pump and the cavitation limit number of revolutions showing the limit of the occurrence of cavitation corresponding to the number of revolutions of the pump and selecting either smaller one of the cavitation limit number of revolutions or the target number of revolutions as the practical target number of revolutions when the target number of revolutions of the water jet pump is input.
The inclusion of a turbocharger (power booster) in a water jet driven personal water craft (jet propulsion boat) can enable rapid acceleration of the personal water craft. However, when engine speed and the number of revolutions of a water jet pump rapidly rise, the flow velocity of a stream flowing in a duct also similarly rapidly rises. This causes a rapid decrease in hydraulic pressure in the duct. When the hydraulic pressure exceeds saturated vapor pressure, bubbles (cavities) are formed at ordinary temperature thereby resulting in cavitation.
FIG. 6 summarizes this problem. In particular, it shows that when a throttle valve (TH) is fully opened, engine (ENG) speed NE accordingly rises. The target boost pressure of the turbocharger also rapidly rises according to the rapid rise of the engine speed and engine speed further rapidly rises. When engine speed or the rate of the rise of engine speed reaches a certain value, cavitation occurs and results in irregular engine speed or hunting. (See a part A in FIG. 6).
In other words, as thrust energy to be originally used for propelling a boat is consumed in vain by the vaporization energy of water, thereby causing vibrations of an impeller of the water jet pump and other parts.
The invention is made to prevent such a situation. The object is to provide a jet propulsion boat that enables preventing hunting by preventing cavitation.